FAQ area is a good place to start: http://www.mp3car.com/vbulletin/faq-emporium/
Also, a CarPC is any PC in a car... So if you have a 200Mhz P2 and put it in your car it is a CarPC. Now that may not be the best choice, but the choices are what you have to read to know what to choose!
Only nice CarPC specific things are really the DC-DC ATX power supplies. Instead of 12vdc of your car to 110vac through an inverter back to 12/5/3.3 dc in ATX powersupply, they go directly from 12v to 12/5/3.3. Saves energy, doesnt induce any alternating noise, and have integrated shut-down controllers so the PC starts and stops (or resume/suspend, hibernate/wake, ect..) as the car does without the need to manually start it each time.